cpython/Doc/whatsnew/3.9.rst

****************************
  What's New In Python 3.9
****************************

:Release: |release|
:Date: |today|

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer in :issue:`12345`.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

This article explains the new features in Python 3.9, compared to 3.8.

For full details, see the :ref:`changelog <changelog>`.

.. note::

   Prerelease users should be aware that this document is currently in draft
   form. It will be updated substantially as Python 3.9 moves towards release,
   so it's worth checking back even after reading earlier versions.


Summary -- Release highlights
=============================

.. This section singles out the most important changes in Python 3.9.
   Brevity is key.


.. PEP-sized items next.



New Features
============



Other Language Changes
======================

* :func:`__import__` now raises :exc:`ImportError` instead of
  :exc:`ValueError`, which used to occur when a relative import went past
  its top-level package.
  (Contributed by Ngalim Siregar in :issue:`37444`.)


* Python now gets the absolute path of the script filename specified on
  the command line (ex: ``python3 script.py``): the ``__file__`` attribute of
  the :mod:`__main__` module, ``sys.argv[0]`` and ``sys.path[0]`` become an
  absolute path, rather than a relative path. These paths now remain valid
  after the current directory is changed by :func:`os.chdir`. As a side effect,
  a traceback also displays the absolute path for :mod:`__main__` module frames
  in this case.
  (Contributed by Victor Stinner in :issue:`20443`.)

* In development mode and in debug build, *encoding* and *errors* arguments are
  now checked on string encoding and decoding operations. Examples:
  :func:`open`, :meth:`str.encode` and :meth:`bytes.decode`.

  By default, for best performance, the *errors* argument is only checked at
  the first encoding/decoding error and the *encoding* argument is sometimes
  ignored for empty strings.
  (Contributed by Victor Stinner in :issue:`37388`.)

* ``"".replace("", s, n)`` now returns ``s`` instead of an empty string for
  all non-zero ``n``.  It is now consistent with ``"".replace("", s)``.
  There are similar changes for :class:`bytes` and :class:`bytearray` objects.
  (Contributed by Serhiy Storchaka in :issue:`28029`.)


New Modules
===========

* None yet.


Improved Modules
================

ast
---

Added the *indent* option to :func:`~ast.dump` which allows it to produce a
multiline indented output.
(Contributed by Serhiy Storchaka in :issue:`37995`.)

asyncio
-------

Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor`
that schedules a shutdown for the default executor that waits on the
:class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also,
:func:`asyncio.run` has been updated to use the new :term:`coroutine`.
(Contributed by Kyle Stanley in :issue:`34037`.)

curses
------

Add :func:`curses.get_escdelay`, :func:`curses.set_escdelay`,
:func:`curses.get_tabsize`, and :func:`curses.set_tabsize` functions.
(Contributed by Anthony Sottile in :issue:`38312`.)

fcntl
-----

Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK`
and :data:`~fcntl.F_OFD_SETLKW`.
(Contributed by Dong-hee Na in :issue:`38602`.)

os
--

Added :data:`~os.CLD_KILLED` and :data:`~os.CLD_STOPPED` for :attr:`si_code`.
(Contributed by Dong-hee Na in :issue:`38493`.)

threading
---------

In a subinterpreter, spawning a daemon thread now raises a :exc:`RuntimeError`. Daemon
threads were never supported in subinterpreters. Previously, the subinterpreter
finalization crashed with a Python fatal error if a daemon thread was still
running.
(Contributed by Victor Stinner in :issue:`37266`.)

venv
----

The activation scripts provided by :mod:`venv` now all specify their prompt
customization consistently by always using the value specified by
``__VENV_PROMPT__``. Previously some scripts unconditionally used
``__VENV_PROMPT__``, others only if it happened to be set (which was the default
case), and one used ``__VENV_NAME__`` instead.
(Contributed by Brett Cannon in :issue:`37663`.)

pathlib
-------

Added :meth:`~pathlib.Path.readlink()` which acts similar to
:func:`~os.readlink`.
(Contributed by Girts Folkmanis in :issue:`30618`)

pprint
------

:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`.
(Contributed by Carl Bordum Hansen in :issue:`37376`.)

importlib
---------

To improve consistency with import statements, :func:`importlib.util.resolve_name`
now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative
import attempts.
(Contributed by Ngalim Siregar in :issue:`37444`.)

Optimizations
=============


Build and C API Changes
=======================

* Add a new public :c:func:`PyObject_CallNoArgs` function to the C API, which
  calls a callable Python object without any arguments. It is the most efficient
  way to call a callable Python object without any argument.
  (Contributed by Victor Stinner in :issue:`37194`.)



Deprecated
==========

* Currently :func:`math.factorial` accepts :class:`float` instances with
  non-negative integer values (like ``5.0``).  It raises a :exc:`ValueError`
  for non-integral and negative floats.  It is now deprecated.  In future
  Python versions it will raise a :exc:`TypeError` for all floats.
  (Contributed by Serhiy Storchaka in :issue:`37315`.)

* The :mod:`parser` module is deprecated and will be removed in future versions
  of Python. For the majority of use cases, users can leverage the Abstract Syntax
  Tree (AST) generation and compilation stage, using the :mod:`ast` module.

* The :mod:`random` module currently accepts any hashable type as a
  possible seed value.  Unfortunately, some of those types are not
  guaranteed to have a deterministic hash value.  After Python 3.9,
  the module will restrict its seeds to :const:`None`, :class:`int`,
  :class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.

* Deprecated the ``split()`` method of :class:`_tkinter.TkappType` in
  favour of the ``splitlist()`` method which has more consistent and
  predicable behavior.
  (Contributed by Serhiy Storchaka in :issue:`38371`.)


Removed
=======

* The abstract base classes in :mod:`collections.abc` no longer are
  exposed in the regular :mod:`collections` module.  This will help
  create a clearer distinction between the concrete classes and the abstract
  base classes.

* The undocumented ``sys.callstats()`` function has been removed. Since Python
  3.7, it was deprecated and always returned :const:`None`. It required a special
  build option ``CALL_PROFILE`` which was already removed in Python 3.7.
  (Contributed by Victor Stinner in :issue:`37414`.)

* The ``sys.getcheckinterval()`` and ``sys.setcheckinterval()`` functions have
  been removed. They were deprecated since Python 3.2. Use
  :func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead.
  (Contributed by Victor Stinner in :issue:`37392`.)

* The C function ``PyImport_Cleanup()`` has been removed. It was documented as:
  "Empty the module table.  For internal use only."
  (Contributed by Victor Stinner in :issue:`36710`.)

* ``_dummy_thread`` and ``dummy_threading`` modules have been removed. These
  modules were deprecated since Python 3.7 which requires threading support.
  (Contributed by Victor Stinner in :issue:`37312`.)

* ``aifc.openfp()`` alias to ``aifc.open()``, ``sunau.openfp()`` alias to
  ``sunau.open()``, and ``wave.openfp()`` alias to :func:`wave.open()` have been
  removed. They were deprecated since Python 3.7.
  (Contributed by Victor Stinner in :issue:`37320`.)

* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread`
  has been removed. It was deprecated since Python 3.8.
  Use :meth:`~threading.Thread.is_alive()` instead.
  (Contributed by Dong-hee Na in :issue:`37804`.)

* Methods ``getchildren()`` and ``getiterator()`` in the
  :mod:`~xml.etree.ElementTree` module have been removed.  They were
  deprecated in Python 3.2.  Use functions :func:`list` and :func:`iter`
  instead.  The ``xml.etree.cElementTree`` module has been removed.
  (Contributed by Serhiy Storchaka in :issue:`36543`.)

* The old :mod:`plistlib` API has been removed, it was deprecated since Python
  3.4. Use the :func:`~plistlib.load`, :func:`~plistlib.loads`, :func:`~plistlib.dump`, and
  :func:`~plistlib.dumps` functions. Additionally, the *use_builtin_types* parameter was
  removed, standard :class:`bytes` objects are always used instead.
  (Contributed by Jon Janzen in :issue:`36409`.)

* The C function ``PyThreadState_DeleteCurrent()`` has been removed. It was not documented.
  (Contributed by Joannah Nanjekye in :issue:`37878`.)

* The C function ``PyGen_NeedsFinalizing`` has been removed. It was not
  documented, tested, or used anywhere within CPython after the implementation
  of :pep:`442`. Patch by Joannah Nanjekye.
  (Contributed by Joannah Nanjekye in :issue:`15088`)


Porting to Python 3.9
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.


Changes in the Python API
-------------------------

* :func:`open`, :func:`io.open`, :func:`codecs.open` and
  :class:`fileinput.FileInput` no longer accept ``'U'`` ("universal newline")
  in the file mode. This flag was deprecated since Python 3.3. In Python 3, the
  "universal newline" is used by default when a file is open in text mode.  The
  :ref:`newline parameter <open-newline-parameter>` of :func:`open` controls
  how universal newlines works.
  (Contributed by Victor Stinner in :issue:`37330`.)

* :func:`__import__` and :func:`importlib.util.resolve_name` now raise
  :exc:`ImportError` where it previously raised :exc:`ValueError`. Callers
  catching the specific exception type and supporting both Python 3.9 and
  earlier versions will need to catch both using ``except (ImportError, ValueError):``.

* The :mod:`venv` activation scripts no longer special-case when
  ``__VENV_PROMPT__`` is set to ``""``.


CPython bytecode changes
------------------------

* The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the
  :keyword:`assert` statement. Previously, the assert statement would not work
  correctly if the :exc:`AssertionError` exception was being shadowed.
  (Contributed by Zackery Spytz in :issue:`34880`.)